Drilling wells for oil and gas production conventionally employs longitudinally extending sections, or so-called “strings,” of drill pipe to which, at one end, is secured a drill bit of a larger diameter. After a selected portion of the bore hole has been drilled, a string of tubular members of lesser diameter than the bore hole, known as casing, is placed in the bore hole. Subsequently, the annulus between the wall of the bore hole and the outside of the casing is filled with cement. Therefore, drilling and casing according to the conventional process typically requires sequentially drilling the bore hole using drill string with a drill bit attached thereto, removing the drill string and drill bit from the bore hole, and disposing and cementing a casing into the bore hole. Further, often after a section of the bore hole is lined with casing and cemented, additional drilling beyond the end of the casing or through a sidewall of the casing may be desired. In some instances, a string of smaller tubular members, known as a liner string, is run and cemented within previously run casing. As used herein, the term “casing” includes tubular members in the form of liners.
Because sequential drilling and running a casing or liner string may be time consuming and costly, some approaches have been developed to increase efficiency, including reamer shoes and drilling with casing. Reamer shoes employ cutting elements on the leading end that can drill through modest obstructions and irregularities within a bore hole that has been previously drilled. Reamer shoes also include an inner section manufactured from a material which is drillable by drill bits. Accordingly, when cemented into place, reamer shoes usually pose no difficulty to a subsequent drill bit to drill through. For instance, U.S. Pat. No. 6,062,326 to Strong et al. discloses a casing shoe or reamer shoe in which the central portion thereof may be configured to be drilled through. However, the use of reamer shoes requires the retrieval of the drill bit and drill string used to drill the bore hole before the casing with the reamer shoe is run into the bore hole.
Drilling with casing employs a drill bit, termed a “casing bit,” attached to the end of the casing string. The casing bit functions not only to drill the earth formation, but also to guide the casing into the bore hole. The casing is, thus, run into the bore hole as it is formed by the casing bit, eliminating the necessity of retrieving a drill string and casing bit after reaching a target depth where cementing is desired. However, in many instances further drilling laterally from the casing or beyond the end of the casing may be desired, requiring drilling through the casing side wall or through or around the casing bit.
Drilling through casing or casing-associated components (casing shoe, casing bit, casing wall, cementing equipment and cement, etc.) may result in damage to the drill bit or tool run into the casing string. Casing as well as casing-associated components often employ iron-based materials in the form of iron-based alloys. Diamond, including specifically polycrystalline diamond compacts, or “PDC's” employed as cutting elements in conventional fixed cutter bits, or “drag” bits, is reactive with iron at high temperatures such as are generated at the cutting edges of such cutting elements during a drilling operation. Therefore, using a conventional drag bit or tool using solely PDC cutting elements to drill through casing or casing-associated components may severely deteriorate the diamond cutting table of the PDC cutting elements, to the extent they are not suitable for further drilling through subterranean formations. This is especially true in high strength alloy steel or “duplex” alloy steel casings. The drag bit or tool must then be retrieved and replaced before drilling resumes.
Special tools known as mills or milling tools have historically been employed in order to drill through casing side wall. Unfortunately, most of these tools are unable to penetrate both the casing sidewall and adjacent subterranean formation effectively to any substantial distance. Therefore, the mill must conventionally be retrieved from the hole and replaced with a drill bit after drilling through the casing side wall is completed. Such a procedure somewhat compromises any time and expense saved by drilling with casing. Several devices have been developed for avoiding damage to the milling tool or the need to retrieve tools used to drill through the casing before drilling any substantial distance into the surrounding formation.
One approach for drilling through casing and casing-associated components includes employing a drill bit or tool having a face on which two different types of cutting elements are disposed. The first type of cutting elements comprise a superabrasive material such as polycrystalline diamond and the second type of cutting elements comprise an abrasive material such as tungsten carbide. The second type of cutting elements exhibit a relatively greater exposure than the first type of cutting elements, so as to engage the interior of the casing or casing-associated components, after which the second type of cutting elements quickly wear away upon engagement with the subterranean formation. Such an approach is disclosed in U.S. Patent Publications 2007/0079995 and 2006/0070771, each of which is assigned to the assignee of the present invention.
One drawback associated with providing two sets of cutting elements on a drill bit or tool is an inability to provide an optimum cutting element layout for drilling the formation after penetration of casing or casing components and surrounding cement. This issue manifests itself not only in problems with attaining an optimum cutting action, but also in problems, due to the presence of the required two sets of cutting elements, with implementing a bit hydraulics scheme effective to clear formation cuttings using drilling fluid when any substantial rate of penetration (ROP) is sought.
To enable effective drilling of casing and casing-associated components manufactured from robust, relatively inexpensive and drillable iron-based material such as, for example, high strength alloy steels which are generally non-drillable by diamond cutting elements as well as enhanced subsequent drilling effectiveness through the surrounding formation, it would be desirable to have a drag bit or tool offering the capability of drilling through such casing or casing-associated components, while at the same time offering the subterranean drilling capabilities of a conventional drag bit or tool employing superabrasive cutting elements.